When writers occasionally ask me how long to make a story, I say, "Until there's no more story." Some are interesting played out long, others get flabby past a range of page count. The treatment given a story—how much is revealed and explored—indicates whether it works best as an extensive tour, just down to the corner, or somewhere in between. Longer length isn't a matter of better, just different.
Let's consider a story that we're all familiar with, A Christmas Carol, and see how it might look in terms of content at various lengths.
Epic Novel (600+ pgs.): Three generations of the down on their luck Scrooge family are chronicled. In narrative description and unfolding of melancholy scenes we're shown how a legacy of hardscrabble circumstances as well as criminally unfair treatment over many years of his grandparents, then parents, then him, have resulted in Ebeneezer's being the distrusting, nasty skinflint that he is. This being ultimately a redemption story, in his sunset years he finds platonic love with a kindly dying dowager, who realizes that Ebeneezer is not really a bad man, just an emotionally wounded one. Over several chapters, her generosity of spirit in her last season dribbles the ice from his heart. He makes a deathbed promise to her that he'll try to see the good in others as she has seen it in him. He gives his saintly but browbeaten office clerk, Bob Cratchet, a sizable raise and all twelve days of Christmas off. It matters less now that Bob's terminally ill son, Tiny Tim, is crippled, since his "Uncle Eb" sweeps him up in his arms at every opportunity, carrying him wherever he wants to go, to view whatever wonders he wants to see for the last year of the lad's life. What about the Ghosts of Christmas Past, Christmas Present, and Christmas Yet to Come? Don't need 'em. No need to rely on the expediency of magic in a single night to defrost the soul of this miserable old geezer. There's the length here for a more realistic story (alas, losing the magic).
Conventional Length Novel (250-400 pgs.): A similar story, except no in-depth coverage of Scrooge's forbears. References to his parents, maybe even a few scenes with his wretched family of origin, but less concentration on Scrooge's background. The story is set mostly in the present, with some expository flashbacks to his formative years.
Novella (90-150 pgs.): Much less detail. Since Scrooge puts up much rationalizing resistance, it takes some time for him to become un-humbugged and absorb what Marley and the three ghosts are trying to teach him. At this length, artificially quick transformation is probably called for.
Long Short Story (40-70 pgs.): As Dickens wrote it.
Conventional Length Short Story (10-20 pgs.): No room for the trio of specters. It's up to a couple of heartrending scenes with, say, Tiny Tim to turn the crusty old bastard around.
Flash Fiction (1-5 pgs.): No fit for any redemption. Scrooge only shown sadly as he is, a wasted life.
If you were writing something on the order of A Christmas Carol you'd need to decide on whether to employ the convenience of wise spirits affecting major change in a person in the course of one night, a chain of events that have similar effect over a longer period of time, or whether to avoid any process of redemption altogether in favor of a starkly tragic tale. Whatever your choice, you'd write your story within the narrative parameters you want...until there's no more story.
Covers of a few of the more than 200 manuscripts into books that I've edited.
